Coherent optical vibration sensors have been investigated for use in numerous applications including strain measurements, equipment diagnostics, medical imaging, and seismic sensing. Various techniques are utilized to observe surface motion. These techniques include, but are not limited to heterodyne laser Doppler vibrometry (e.g., U.S. Pat. No. 4,834,111 A Khanna et al.) for observation of surface velocity in the axial dimension of the interrogation beam, shearography (e.g., U.S. Pat. No. 5,011,280 A Hung) for observation of the gradient of displacement in two dimensions, electronic speckle pattern interferometry (e.g., U.S. Pat. No. 4,018,531 A Leendertz) for in-plane or out of plane displacement or out of plane displacement gradients, and speckle pattern imaging for out of plane displacements. The techniques listed are generally used for observation of one or two degrees of freedom. Variations using multiple coherent beams (e.g., U.S. Pat. No. 7,242,481 B2 Shpantzer et al.) have been used to observe three degrees of freedom.
It is of interest to simultaneously observe three-degrees of freedom using a single coherent beam.
The current invention combines elements of heterodyne Doppler vibrometry and digital speckle photography along with signal processing routines to simultaneously observe velocity in the axial dimension of the interrogation beam and out of plane tilts of the illuminated region. This provides the ability to observe three-degrees of freedom using a single coherent beam.